Mercury (Hg) is a toxic heavy metal occurring in several physical and chemical forms. Elemental mercury (Hg⁰) emitted to the atmosphere is converted to soluble forms, deposited into soil and water, and methylated. Methylmercury (MeHg) bioaccumulates up the aquatic food chain and reaches the human diet.^1,2 Fish and dental amalgam are two major sources of human exposure to organic and inorganic Hg, respectively.

Even when dental mercury amalgam (at present about 50% mercury by weight) was introduced more than 150 years ago there was concern about its toxicity. After several “amalgam wars”, the safety of dental amalgam was established, however, and no uptake of mercury was supposed to occur from amalgam fillings. The debate was revived more than two decades ago in Europe, and then in North America. In the 1980s several studies confirmed that dental amalgam is a significant source of exposure to mercury in humans, mainly by inhalation of Hg⁰.^1,2 New assessments of exposure and risk were made by national bodies.^3,4 Usually the conclusions were that dental amalgam is a source of low level exposure to mercury, but there is no evidence of adverse health effects at these levels. In some countries strong anti-amalgam groups have formed, and policy has changed, aiming at decreasing or abolishing the use of dental amalgam.

The scientific discussion has focused on two questions: What is the dose? Could it cause symptoms and/or disease?

HOW MUCH?

In this issue, Dye and coworkers present recent data on urinary mercury (U-Hg) in a representative sample of about 1600 US women aged 16–49 years from the NHANES study of 1999–2000.⁵ This is an important piece of information on …